[swift-users] Function to unsafe pointer and back

[Lou Zell]

On Thu, Sep 1, 2016 at 1:48 PM, Andrew Trick <atrick at apple.com> wrote:

>
> Also, in the first case the UnsafePointer points to local memory that
> holds the function value. In the second case, you’re trying to load a
> function value from the address of the function body.
>
>
Beginning to see...

On Fri, Sep 2, 2016 at 9:49 AM, Jordan Rose <jordan_rose at apple.com> wrote:

>
> I feel like there’s still one last piece missing here, which is that in
> Swft a "function-typed value” is not the same as a “function pointer” in C.
> Because a function value might have come from a closure, it might include
> captured state…so the representation of a closure is not the same as a
> representation of a plain function.
>
> Because of *that*, the address you’re seeing isn’t the address of the
> function in memory; it’s the address of the temporary allocation used to
> represent “this global function and no associated state”. That’s why you
> can’t just form a pointer with that bit-pattern and expect it to work.
>
>
Seeing!


> Hope you’ve gotten some useful information between the three of us. :-)
>

That's the trifecta of support right there!  Thank you all!

I ran a new little experiment for the curious:

var a: () -> () = {
    print("foo!")
}
a: () -> () = 0x0000000100561080 $<snip> at repl2.swift
withUnsafePointer(&a) {$0}
$R0: UnsafePointer<() -> ()> = 0x00007fff5fbffce8

UnsafePointer<() -> ()>(bitPattern: 0x00007fff5fbffce8)!.pointee()
// foo!
UnsafePointer<() -> ()>(bitPattern: 0x0000000100561080)!.pointee()
// Crashes as expected!

And:

typealias MyType = @convention(c) () -> ()
var b: MyType = {
    print("bar!")
}
b: __lldb_expr_10.MyType = 0x000000010056b150 $<snip> at repl12.swift
withUnsafePointer(&b) {$0}
$R1: UnsafePointer<__lldb_expr_10.MyType> = 0x0000000100564c80

UnsafePointer<MyType>(bitPattern: 0x0000000100564c80)!.pointee()
// bar!
UnsafePointer<MyType>(bitPattern: 0x000000010056b150)!.pointee()
// Crash!

I am surprised to find here that the addresses are different, albeit much
closer in space than the first example. I suppose there's still some swift
machinery at work to make a function type declared as @convention(c) to
_behave_ as a function pointer.  Cool.

Thanks again Quinn, Andy, Jordan!
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